By far, the majority of nuclear power reactors are water cooled and moderated reactors, utilizing enriched uranium dioxide as fuel. The core of the reactor is formed by elongated fuel rods which are grouped into bundles which are generally square in cross section. The rods have diameters usually in the range of one-fourth to one half inch and may be ten or twelve feet long. They are held parallel and closely spaced from each other. Each rod is formed of a jacket or "cladding" made of zirconium alloy or stainless steel, which is filled with the uranium dioxide. Most commonly, the uranium dioxide is in the form of pellets which are just enough smaller than the cladding to slide in conveniently. The uranium dioxide may, on the other hand, be in the form of microspheres or granules which are compacted within the cladding. The remainder of the space within the cladding is commonly filled with helium, which has a high thermal conductivity. The helium is frequently under higher than atomospheric pressure.
During the operation of the reactor, holes may develop in the cladding due to stress, corrosion, wear, or defective welding to the end plugs which close the ends of the cladding tubes. If this happens, the helium and fission gases will escape into the cooling water of the reactor and the water will enter the cladding tubes.
After a given fuel assembly has been exposed in the reactor for a given length of time, it is taken out, checked for defects, repaired if necessary, and either returned to the reactor or sent for reprocessing or permanent storage. If the assembly is to be returned to the reactor, it is almost essential that it be checked for defective fuel rods. These irradiated assemblies are highly radioactive and must be stored and inspected under water in order to remove heat caused by the decay of fission products as well as to protect persons working with them. It is therefore highly desirable to provide a method of testing fuel rods for leaks while they are assembled and underwater. One method of doing so is by ultrasonic testing. Such a method is disclosed in U.S. Pat. No. 4,313,791, granted Feb. 2, 1982 and assigned to the Babcock and Wilcox Company. In this method, a transducer emitting ultrasonic vibrations is placed against a fuel rod and an ultrasonic beam is transmitted into the rod by the transducer. The test is performed on a portion of the fuel rod which does not contain uranium dioxide. An analysis of the waves received by a pulse-echo system, reveals whether or not this portion of the rod is filled with water. It is disclosed as carried out at the lower plenum of a fuel rod and apparently would not be operative in a portion of a rod where uranium dioxide is present.
A weakness of this method lies in the fact that many, probably most, fuel rods do not have a lower plenum.